A non-metallic inclusion in steel, such as alumina, which is deposited on the inner hole of an immersion nozzle during a steel continuous casting process, has adverse affects on the quality of a final product. In connection with strict quality recently required for steel products, a good deal of effort has been made to reduce the deposit of the non-metallic inclusion. Moreover, in the casting for long hours, the deposit on the inner hole of a nozzle, such as an immersion nozzle, causes clogging of the inner hole, which leads to difficulty in continuing the casting operation or deterioration in productivity.
Heretofore, in order to physically reduce the deposit such as alumina, argon gas has been injected from the inner surface of a nozzle into molten steel. In this technique, if the argon gas is injected at an excessive amount, resulting gas bubbles will be incorporated into slabs to cause the formation of pinholes in a mold or defect in a steel product. Thus, the gas has to be injected at a limited amount, and consequently an adequate effect cannot be always obtained.
There has also been known a technique for chemically reducing the deposit such as alumina, which is intended to provide a function of preventing alumina deposit (anti-alumina-deposit function) to a refractory material of an immersion nozzle. A typical refractory material according to the above technique is disclosed in Japanese Patent Publication No. 02-23494. This refractory material is based on CaO—ZrO2-Graphite (hereinafter referred to as “ZCG”) using CaO—ZrO2 clinker which contains CaZrO3 as a primary component, and the publication discloses the application of the ZCG-based material to the inner hole of an immersion nozzle.
Currently, the ZCG-base material has put into practical use only in a part of nozzles, but not come into general use. One of the reasons is in that if steel has a relatively large amount alumina inclusion, an adequate anti-alumina-deposit effect cannot be obtained. Thus, in such a case, it is required to take a measure of increasing the amount of CaO bringing out the anti-alumina-deposit effect, or of reducing the amount of carbon or graphite so as to increase the ratio of CaO to graphite. However, if the amount of CaO is increased, the ZCG-base material will inevitably have a larger thermal expansion coefficient. Thus, the material applied to the body of a nozzle is likely to cause thermal spalling. Even if the material is applied to only an inner hole portion of a nozzle, the body of the immersion nozzle will be likely to be pressed and cracked by the thermally expanded material.
Japanese Patent Laid-Open publication No. 62-24846 discloses a continuous casting nozzle having a cylindrical sleeve made of lime-based refractory material and inserted into the inner hole surface thereof, which exhibits excellent absorbability of alumina to prevent clogging of the inner hole thereof. As with the aforementioned publication, this nozzle has a problem of cracks of the nozzle body caused by expansion of the refractory sleeve due to a relatively large thermal expansion coefficient of the lime-based refractory material.